1. Field of the Invention
The present invention relates to aluminum nitride ceramics having a low volume resistivity, a member for use in a semiconductor producing system, a corrosion resistant member and a conductive member.
2. Related Art Statement
Aluminum nitride sintered bodies have an excellent corrosion resistant property against various corrosive substances such as a halogen-based gas and have thus been utilized for various members in a system for producing semiconductors, including ceramic heaters and electrostatic chucks. A conventional dense aluminum nitride sintered body has a volume resistivity higher than 1013 Ω·cm at room temperature. It is therefore needed to reduce the volume resistivity of the aluminum nitride sintered body depending on intended applications. The applicant filed patent applications for the following aluminum nitride sintered bodies each having a volume resistivity at room temperature as low as about 1010 Ω·cm:                (1) A sintered body with a small amount of Y2O3 added (Japanese patent publication 9-315, 867A); and        (2) A sintered body with a small amount of CeO2 added (Japanese patent publication 2001-163, 672A).        
The above aluminum nitride sintered bodies have a high purity of aluminum nitride and a low volume resistivity at the same time, and thus may be utilized for various applications including a base material for an electrostatic chuck.
On the other hand, it has been known to reduce the resistivity of an aluminum nitride sintered body by adding a large amount (for example, not lower than 30 volume percent) of a conductive ceramic material such as silicon carbide and titanium nitride to provide a composite ceramic.
Further, in some applications, such as a base material for an electrostatic chuck, the base material is used in a wider temperature range. It is thus demanded to provide a material with a reduced dependency of the volume resistivity on temperature change. The applicant filed US-2002-0110709-A1 and disclosed an aluminum nitride sintered body having a volume resistivity at room temperature of 108 to 1012 Ω·cm and a change of volume resistivity between 25 to 400° C. of the order of about 103. The sintered body may be produced by adding samarium oxide into a powdery raw material of aluminum nitride. Japanese patent publications 8-314953A and 8-350075A disclose a method for further reducing the dependency of volume resistivity on temperature of an aluminum nitride sintered body. In the disclosure, a conductive ceramics having a metallic conductivity such as TiN is added into powdery raw material of aluminum nitride in an amount of 10 to 30 weight percent.
In methods described in Japanese patent publication 9-315867A and 2001-163, 672A, it is possible to reduce the volume resistivity of aluminum nitride ceramics at room temperature to a value as low as 108 to 109 Ω·cm by adjusting a composition of starting material and sintering conditions. The requirements for the composition and sintering conditions are, however, strictly limited to reduce the volume resistivity at a value not higher than 1010 Ω·cm.
Further, in Japanese patent publication 5-178, 671A, boron carbide (B4C) in an amount of 0.5 weight percent calculated as boron is added into aluminum nitride powder and the mixed powder is then sintered at 1800° C. for two hours. It is thus obtained a sintered body having a thermal conductivity of 153 W/m·K and a relative porosity of 98.9 percent. Boron carbide is added as a second additive for helping the sintering process of aluminum nitride powder at atmospheric pressure. The addition of boron carbide is said to have the effects of reducing the crystalline growth. A sintered body having a high thermal conductivity is thereby obtained having an improved surface roughness and reduced color defects or irregularities. The relationship between the addition of boron carbide and volume resistivity, however, is not described. According to the study of the inventors, the inventive ceramics cannot be produced according to the conditions and composition disclosed in the patent.
In Japanese patent publications 8-314953A and 8-350075A, conductive ceramics are added into an aluminum nitride raw material to provide a composite ceramic. According to this method, it is necessary to add a large amount of conductive ceramics into the aluminum nitride raw material to obtain the composite ceramic to sufficiently reduce the volume resistivity of the sintered body. Characteristic properties of aluminum nitride are, however, lost by adding a large amount of conductive ceramics. For example, a high thermal conductivity, low thermal expansion, the corrosion resistant property and the chemical stability of aluminum nitride are either lost or reduced. It is therefore desired to preserve the characteristic properties of aluminum nitride and reduce its volume resistivity.